In the quest for hydrocarbons, the need can arise for drilling of an earth borehole in a determined spatial relationship with respect to another existing borehole. One example is the so-called steam-assisted gravity drainage (“SAGD”) process which is used to enhance production from an existing section of a generally horizontal production wellbore in a reservoir of high viscosity low-mobility crude oil. A second wellbore, to be used for steam injection, is drilled above and in alignment with the production wellbore. The injection of steam in the second wellbore causes heated oil to flow toward the production well, and can greatly increase recovery from the reservoir. However, for the technique to work efficiently, the two boreholes should be in good alignment at a favorable spacing over the length of the production region.
Referring to FIG. 1, a pair of SAGD wells 10 and 20 are shown in the process of being constructed. The lower well is drilled first and then completed with a slotted liner in the horizontal section. The lower well 10 is the producer well and is located with respect to the geology of the heavy oil zone. Typically, the producer well is placed near the bottom of the heavy oil zone. The second well 20 is then drilled above the first well, and is used to inject steam into the heavy oil formation. The second, injector well is drilled so as to maintain a constant distance above the producer well throughout the horizontal section. Typically, SAGD wells are drilled in Canada to maintain a vertical distance of 5±1 meters above the horizontal section, and remain within ±1 meters of the vertical plane defined by the axis of the producer well. The length of the horizontal section can typically vary from approximately 500 meters to 1500 meters in length. Maintaining the injector well precisely above the producer well and in the same vertical plane is beyond the capability of conventional MWD direction and inclination measurements.
Instead, magnetic ranging is typically used to determine the distance between the two wells and their relative position. In U.S. Pat. No. 5,485,089, a magnetic ranging method is described where a solenoid is placed in one well and energized with current to produce a magnetic field. This solenoid (e.g. 12 in FIG. 1, which also depicts magnetic field B) comprises a long magnetic core wrapped with many turns of wire. The magnetic field from the solenoid has a known strength and produces a known field pattern that can be measured in the other well, for example by a 3-axis magnetometer (represented at 21 in FIG. 1) mounted in a measurement while drilling (MWD) tool. The solenoid must remain relatively close to the MWD tool for the magnetic ranging. The solenoid is pushed along the horizontal section of the well using a wireline tractor (e.g. 14 in FIG. 1), or coiled tubing, or it can be pumped down inside tubing (not shown).
In a typical sequence of operations, the bottom hole assembly (BHA) in the second well drills ahead a distance of 10 m to 90 m, corresponding to one to three lengths of drill pipe. The distance between measurements depends on the driller's ability to keep the well straight and on course. The drilling operation must be halted to perform the magnetic ranging operation. U.S. Pat. No. 5,485,089 teaches that first, the 3-axis magnetometers in the MWD tool measure the (50,000 nTesla) Earth's magnetic field with the current in the solenoid off. Then the solenoid is activated with DC current to produce a magnetic field which adds to the Earth's magnetic field. A third measurement is made with the DC current in the solenoid reversed. The multiple measurements are made to subtract the Earth's large magnetic field from the data obtained with the solenoid on.
The solenoid is then moved to a second position along the completed wellbore by a tractor or by other means. If the first position is slightly in front of the MWD magnetometer (i.e. closer to the toe of the well), then the other position should be somewhat behind the MWD magnetometer (i.e. closer to the heel of the well). The solenoid is again activated with DC current, and the MWD magnetometers make the fourth measurement of the magnetic field with DC current. The DC current in the solenoid is then reversed, and a fifth measurement is made. The five magnetic field measurements are transmitted to the surface where they are processed to determine the position of the MWD tool magnetometers with respect to the position of the solenoid.
There are drawbacks to this process. First, the solenoid must be physically moved between the two borehole positions, during which time the BHA is not drilling. This movement requires that the tractor be activated and driven along the wellbore, which is time consuming. Second, any errors in measuring the two axial positions of the solenoid, or errors in the distance the solenoid moves, introduce errors in the calculated distance between the two wells. Third, since the solenoid is driven from one position to another, the distance the solenoid travels may vary from one magnetic ranging operation to the next. Since the MWD tool does not know how far the solenoid moved, it cannot compute the distance to the first well. This means that all five magnetic field measurements must be transmitted to the surface via the typically slow MWD telemetry system. Only after the MWD measurements have been decoded at the surface and the appropriate algorithms processed (including knowledge of the two solenoid positions), can the distance between the two wells be determined and drilling resumed. Hence, this magnetic ranging process results in excess rig time and thus increases the cost of drilling the well.
Reference can also be made to U.S. Pat. Nos. 3,731,752, 4,710,708, 5,923,170 and Re. 36,569, and also to Grills et al, “Magnetic Ranging Technologies for Drilling Steam Assisted Gravity Drainage Wells Pairs and Unique Well Geometries”. SPE 79005, 2002, and to “Kuckes et al., New Electromagnetic Surveying/Ranging Method for Drilling Parallel, Horizontal Twin Wells,” SPE 27466, 1996.
It is among the objects of the present invention to provide improved magnetic ranging and improved distance and direction determination between wellbores and to improve controlled drilling of an earth borehole in a determined spatial relationship with respect to another existing earth borehole.